A. Field of the Invention
The present invention relates to a lock-up piston in a torque converter, and more particularly to a piston that includes a disc-shaped portion and an annular tube-shaped portion formed on an outer periphery of the disc-shaped portion, the annular tube-shaped portion extending in an axial direction.
B. Description of the Related Art
A torque converter is a device which has three types of internal vaned wheels: an impeller, a turbine and a stator. Torque converters typically configured to transmit torque via movement of hydraulic fluid within the device. A torque converter typically includes a front cover that is fixed to the impeller. The impeller typically includes an impeller shell welded to the front cover, and a plurality of impeller blades. The front cover is coupled to a torque producing mechanism, such as the crankshaft of an engine. The turbine is typically coupled to a transmission shaft. Torque is transmitted from the front cover to the impeller, the blades of the impeller urge the hydraulic fluid toward the turbine causing the turbine to rotate, and the turbine causes the transmission shaft to rotate.
A lock-up clutch within the torque converter is disposed between the turbine and the front cover. The lock-up clutch is typically coupled to the turbine for rotation therewith. When the lock-up clutch is engaged with the front cover, torque is directly transmitted from the front cover to the turbine.
Usually, the lock-up clutch includes a piston that can be brought into contact with the front cover in response to fluid pressure changes. The piston typically includes an annular flat portion on an outer peripheral portion thereof, the annular flat portion having an annular shaped frictional facing fixed thereto. A retaining plate is fixed to a portion of the piston for supporting a torsion spring that elastically couples the piston to a driven plate that is connected to the turbine.
The piston is selectively moved in an axial direction in and out of engagement with the front cover by changes in hydraulic fluid pressure within portions of the torque converter. When the annular shaped frictional facing fixed on the outer periphery of the piston is in direct contact with a corresponding frictional surface of the front cover, torque from the front cover is transmitted through the lock-up clutch to the turbine.
Preferably, the frictional surface of the front cover should be flat. In reality, however, undulations and irregularities caused by, for instance, welding, sometimes develop in the frictional surface. With such undulations present, when the friction facing of the piston is in contact with the frictional surface of the front cover, the rigidity of the piston prevents complete planar surface to surface contact between the frictional surface on the front cover and the frictional facing on the piston. Specifically, only the undulated portions of frictional surface of the front cover closest to the piston make contact with the frictional facing on the piston. As a result, the performance of the lock-up clutch is degraded and slippage of the lock-up clutch may occur. Therefore, under such circumstances, the lock-up clutch is unable to operate properly and may not sufficiently absorb or dampen vibrations during slippage of the lock-up clutch, and further may not transmit torque to the turbine in an efficient manner due to the slippage.
Further, in a known configuration of a piston of a lock-up clutch, the piston is formed on an outer radial periphery thereof with an annular rim or tubular portion which restricts radially outward movement of torsion springs. Typically the torsion springs are moved radially outward by centrifugal force. Therefore, such an annular rim or tubular portion is necessary to retain the torsions springs. Unfortunately, such an annular rim or tubular portion provides the piston with even greater rigidity, thereby further adding to the problems discussed above.